Re: [patch 4/8] x86/entry: Move irq tracing on syscall entry to C-code

Thomas Gleixner
2020-02-26T19:51:20Z

Andy Lutomirski <luto@amacapital.net> writes:
>> On Feb 26, 2020, at 12:17 AM, Peter Zijlstra <peterz@infradead.org> wrote:
>> ﻿On Tue, Feb 25, 2020 at 09:43:46PM -0800, Andy Lutomirski wrote:
>>> Your earlier patches suggest quite strongly that tracing isn't safe
>>> until enter_from_user_mode(). But trace_hardirqs_off() calls
>>> trace_irq_disable_rcuidle(), which looks [0] like a tracepoint.
>>>
>>> Did you perhaps mean to do this *after* enter_from_user_mode()?
>>
>> aside from the fact that enter_from_user_mode() itself also has a
>> tracepoint, the crucial detail is that we must not trace/kprobe the
>> function calling this.
>>
>> Specifically for #PF, because we need read_cr2() before this. See later
>> patches.
>
> Indeed. I’m fine with this patch, but I still don’t understand what
> the changelog is about.

Yeah, the changelog is not really helpful. Let me fix that.

> And I’m still rather baffled by most of the notrace annotations in the
> series.

As discussed on IRC, this might be too broad, but then I rather have the
actual C-entry points neither traceable nor probable in general and
relax this by calling functions which can be traced and probed.

My rationale for this decision was that enter_from_user_mode() is marked
notrace/noprobe as well, so I kept the protection scope the same as we
had in the ASM maze which is marked noprobe already.

Thanks,

tglx